1. Field of the Invention
The present invention relates to a method and machine for fabricating electroluminescent devices which can be used to provide emissive segments or emissive matrix displays or which can be used as display devices in various intelligent terminals.
2. Related Arts
As is well known in the art, an electroluminescent device makes use of the fact that when an electric field is applied to a phosphor as consisting of zinc sulfide (ZnS), the phosphor emits light. Electroluminescent devices of this kind have attracted attention as devices forming an emissive flat-panel display.
The cross-sectional structure of a typical conventional electroluminescent device is shown in FIG. 33, where the device is generally indicated by numeral 10. This electroluminescent device 10 is fabricated by successively forming a first electrode layer 2, a first dielectric layer 3, a luminescent layer 4, a second dielectric layer 5, and a second electrode layer 6 on an insulating glass substrate 1. The first electrode layer 2 is made of a film of ITO (indium tin oxide) which is optically transparent. The first and second dielectric layers 3 and 5, respectively, are made of tantalum pentoxide (Ta.sub.2 O.sub.5) or other similar material. The second electrode layer 6 is made of a film of ITO. These films of ITO are conductive films of indium oxide (In.sub.2 O.sub.3) doped with tin (Sn). Since these ITO films have low resistivities, they have enjoyed wide acceptance as transparent electrode layers.
The color emitted by the luminescent layer 4 is determined by the kind of the dopant incorporated in the zinc sulfide (ZnS). For example, where terbium (Tb) is used as the luminescent center, the luminescent layer emits yellowish-green color. Where samarium (Sm) is added, reddish-orange color is produced. Where thulium (Tm) is added, blue color is emitted.
Some researchers are discussing the possibility that zinc sulfide (ZnS) doped with thulium (Tm) and strontium sulfide (SrS) doped with cerium (Ce) are adopted as the materials of the luminescent layer 4 emitting blue light, the layer 4 being contained in the electroluminescent device 10 of the structure described above.
Where the above-described zinc sulfide (ZnS) doped with thulium (Tm) is used in the luminescent layer 10, the emission brightness is so low that a practically sufficient brightness is not obtained. Where the strontium sulfide (SrS) doped with cerium (Ce) is employed, the emission brightness is relatively high but the emitted light is blue-green. Therefore, in order to obtain blue light, it is necessary to use a filter which cuts off light having wavelengths of, for example 500 nm or more. If such a filter is exploited, the resulting emission brightness is only about ten percent of the original emission brightness. For these reasons, it is difficult to use this structure as a blue light-emitting device providing practically sufficiently intense brightness. In consequence, a high-quality luminescent layer of strontium sulfide (SrS) doped with cerium (Ce) is needed to obtain a practically sufficient brightness.
In "Digest of Technical Papers on 1993 Display Information Society International Meeting, pp. 761-764", use of strontium thiogallate (SrGa.sub.2 S.sub.4), barium thiogallate (BaGa.sub.2 S.sub.4), and calcium thiogallate (CaGa.sub.2 S.sub.4) as host materials of a luminescent layer is being discussed. Adoption of a luminescent layer consisting of such a host material to which cerium (Ce) is added as the luminescent center is being discussed. Use of this kind of luminescent layer can shift the emission spectrum toward shorter wavelengths. For example, even where green component is cut off by the use of a filter, the loss of the emission brightness caused by this filter can be reduced greatly.
However, as described in the aforementioned paper and in Japanese Patent Laid-Open No. 65478/1993, the luminescent layer is formed by sputtering during the fabrication. During this sputtering process, a thermal treatment is required to be performed at a high temperature exceeding 650.degree. C. after the formation of the luminescent layer in order to crystallize the host material of the luminescent layer.
As a result, the following difficulties occur.
First, at such a high temperature, a strain is produced in the glass substrate. Furthermore, limitations are imposed on materials from which electrodes are fabricated.
Even if the thermal treatment is short, the crystallization of the host material of the luminescent layer does not progress sufficiently provided that the thermal treatment temperature lies within the temperature range in which neither the glass substrate nor the electrodes are adversely affected. The result is that a luminescent layer of high crystallinity cannot be obtained. This makes it impossible to realize high-brightness emission of blue light.
Moreover, since the film consists of ternary compounds, the composition of the film tends to deviate from the desired composition during thermal treatment. This deviation detrimentally affects the emission brightness.
Known methods for forming high-quality luminescent layers for electroluminescent displays include metal organic chemical vapor deposition (MOCVD) and atomic layer epitaxy (ALE). Where luminescent layers are formed based on the above-described zinc sulfide (ZnS) by these methods, electroluminescent devices of high emission brightness can be obtained. Strontium sulfide (SrS) doped with cerium (Ce) and grown by ALE is known as one material of the aforementioned luminescent layer emitting blue light.
However, the ALE has an intrinsic disadvantage that the growth rate is slow. Therefore, it takes a very long time to form a luminescent layer. Consequently, it is generally desired to form a luminescent layer by the MOCVD which enables a high growth rate.
However, if an alkaline earth sulfide such as strontium sulfide (SrS) doped with cerium (Ce) is grown by MOCVD, gaseous source materials react with each other in gas phase within a reaction furnace, producing particles and subsidiary products. As a result, it is impossible to derive a high-quality luminescent layer.
In the case of a ternary-compound luminescent layer such as calcium thiogallate (CaGa.sub.2 S.sub.4), the problems are complicated further. For this reason, it has been considered that it is impossible to form a ternary-compound luminescent layer such as calcium thiogallate (CaGa.sub.2 S.sub.4) by MOCVD.